Lipid storage is critical for metabolic homeostasis, and influences components ofthe metabolic syndrome, including visceral obesity, insulin resistance, and dyslipidemia. The objectives are to further elucidate the function of lipin-1 in lipid synthesis, storage, and lipid signaling in adipose tissue and muscle, and to identify novel genes that influence adipose tissue mass and function. Our previous studies demonstrated that lipin-1 is a determinant of adipose tissue development, insulin sensitivity, and energy metabolism. Lipin-I is a phosphatidate phosphatase (PAP) enzyme that converts phosphatidate to diacylglyerol, and accounts for all PAP activity in adipose tissue and muscle. In addition, it is a transcriptional coactivator that influences expression of lipid metabolism genes in liver. The specific aims are: (1) Determine how lipin-1 modulation of phosphatidate levels regulates adipogenesis and influences insulin sensitivity in skeletal muscle. In the absence of lipin-1, phosphatidate accumulates in tissues, which may activate signal transduction pathways and/or alter mitochondrial or ER membrane properties. We hypothesize that phosphatidate levels determined by lipin-1 influence expression of PPARgamma and adipocyte differentiation, and insulin sensitivity in muscle. We will investigate how dysregulation of lipin-1 and phosphatidate levels contribute to altered metabolism in adipose tissue, muscle, and liver. (2) Evaluate the role of lipin-1 in statin-induced myotoxicity. Human LPINI nonsense mutations cause childhood myopathy, and missense mutations have been associated with statin-induced myopathy. We will test the hypothesis that impaired lipin-1 activity and statin action interact to impair mitochondrial function. We will functionally characterize mutant lipin-1 proteins, evaluate effects of lipin-1 deficiency on statin-induced myotoxicity in the mouse, and evaluate a cohort of subjects with statin-induced myopathy for LPINI mutations. (3) Identify and characterize the molecular function of novel adiposity genes. We hypothesize that genetic variations that alter adiposity in vivo will reveal novel genes in adipose tissue function. We will investigate the function of 7 candidate genes identified by network modeling in the mouse for roles in adipocyte function using in vitro and in vivo methods.